Caecidotea stygia Packard 1871

Caecidotea stygia Packard

Caecidotea stygia Packard, 1871:751,752, figs. 132,133.—Smith, 1874:661; 1875:477.—Hubbard, 1880:37, fig. 10.—Stebbing, 1893:377.—Richardson, 1901:553; 1905:434, 435, figs. 490, 491a,b, 492a,g.—Hay, 1902a:225,226; 1902b:421–424, 426–428, figs. 4, 5a,g.—Ulrich, 1902: 93.—Banta, 1907:76,77; 1910a:246–248, 251,252, 269–280, 283,284, 292–301, 305–308; 1910b:439–488.—Stafford, 1911:575.—Fowler, 1912:522.—Pratt, 1916:377.—Racovitza, 1920:99; 1923:107.—Hungerford, 1922:175, 176.—Chappuis, 1927:61.—Bolivar and Jeannel, 1931:299, 300, 302, 308.—Creaser, 1931:5.—Giovannoli, 1933a:621; 1933b:239.—Hoffman, 1933:26.—Van Name, 1936: 32, 466–468, 470–473, fig. 293; 1942:317.—Park et al., 1939:120–125.—Mackin and Hubricht, 1940:383–385, 394,395.—Dearolf, 1953:227.—Nicholas, 1960a:132; 1960b:51,52.—Holsinger, 1963:29; 1976:75.—Steeves, 1969:51,52.—Holsinger and Steeves, 1971:195.—Bowman and Beckett, 1978:294–302, figs. 1–6.—Peck and Lewis, 1978:45, 54,55.

Caecidotea microcephala.—Cope, 1872a:409, 411, 417–420, fig. 109; 1872b:161, 163, 171, 174,175, figs. 109,110; 1878:492, 495–496, 505, fig. 109.

Caecidotea.—Smith, 1873:244,245.—Hobbs et al., 1977:104.

Caecidotaea stygia.—Packard, 1873:95,96; 1885b:3, 8–10, 14; 1888:10–12, 14–16, 19, 24, 29–33, 82, 86, 108–110, 118, 122, 142, 150, 151, pl. 3: figs. 1–8, pl. 4: figs. 1, 2, pl. 23: fig. 1 in part.

Asellus stygius.—Underwood, 1886:359.—Hay, 1891:150 [partim].—Miller, 1933:100–102.—Racovitza, 1950:164–176, figs. 1–13.—Pennak, 1953:433,434, fig. 272a.—Chappuis, 1955:164; 1957:38.—Cole, 1959:74.—Mackin, 1959: 875.—Barr, 1961:32.—Minckley, 1961:452–455; 1963:47, 49, 74, 102, table 12.—Poulson, 1963:264; 1964:753, 755, 763.—Steeves, 1963a:470–474, 476, figs. 2–6; 1963b:462; 1965:82,83; 1966:394–396, 401, fig. 7.—Vandel, 1964:159, 461, 496, 512; 1965:127, 391, 419, 433.—Clifford, 1966:71, 90.—Krekeler and Williams, 1966:394.—Barr, 1967:160, 188, 190, 192; 1968:60, 92.—Minckley and Cole, 1968:2.—Steeves and Holsinger, 1968:82.—Holsinger, 1969:26; 1976:75.—Seidenberg, 1969:52, 81.—Williams, 1970:1.—Fleming, 1972a:227, 230,231, 249–252; 1972b: 498; 1973:297.—Hobbs, 1973a:11.—Page, 1974:91.—Barr and Kuehne, 1971:70,71, 85.—Craig, 1975:4.

Cecidotaea stygia.—Cope and Packard, 1881:879,880.—Packard, 1885a:85,86.

Caecidotea (Asellus) stygia.—Garman, 1892:240.—Packard, 1894:729,730, 742.

Caecidotaea stygia.—Chilton, 1894:175,176, 252.—Blatchley, 1896:127, 133, 135, 142, 149, 174,175, 207,208.

Caecidothea stygia.—Racovitza, 1925:580–582, figs. 196,197, 200, 201.

Asellus stygia.—Pratt, 1935:439.—Dearolf, 1942:50.

Asellus stygeus.—Hubricht, 1950:16.

Asellus (Caecidotea) stygius.—Chappuis, 1950:179–182, figs. 1–3.

Asellus (Conasellus) stygia.—Birstein, 1951:53, 111.

Asellus (Proasellus) stygius.—Bresson, 1955:51.

isopods.—Barr, 1964:79.

Asellus (= Caecidotea) stygia.—Eberly, 1966:286.

isopod.—Mohr and Poulson, 1966:94,95 [illustration].

Conasellus stygius.—Henry and Magniez, 1970:337, 356, 359.

Asellus, of. stygius.—Cooper and Beiter, 1972:880.

Caecidotea (= Asellus).—Hobbs and Barr, 1972:37.

Asellus styguis.—Fleming, 1973:294.

“isopod”.—Hobbs, 1973b: unnumbered [tables 3 and 4].

[For misidentifications of Caecidotea kendeighi as C. stygia, see synonymy of former.]

HISTORY.—The first appearance of Caecidotea stygia in the literature was Packard's (1871) description of the species in an article on the inhabitants of Mammoth Cave, Edmonson Co., Kentucky. Packard felt that this isopod was related to the marine genus Idotea, differing from Idotea in being eyeless, possessing an 8-merous antenna 1 (instead of 4) and an enlarged head, and lacking a segmented abdomen. The description was accompanied by 2 habitus drawings and single drawings of pereopod 1 and antenna 1, none of which were sufficiently detailed to distinguish the species adequately. Packard apparently described the species from a single specimen lacking the second antennae and uropods, and from his indication of the size of the species (“Length .25 inch”), his specimen was probably immature. Both the new genus and species were described in two paragraphs, but this description was concerned primarily with the relative sizes of body segments and the number of segments in the antennae, which likewise do not serve to separate C. stygia adequately from other species.

Cope (1872a) discussed the fauna of Wyandotte Cave, Crawford Co., Indiana, describing Caecidotea microcephala from a cave near Wyandotte Cave (Saltpeter Cave) and comparing it to C. stygia. Cope's description included a tiny illustration of the mandible and a curious drawing of an entire specimen bearing caudal egg sacs rather than uropods. Cope felt justified in describing the new species because of differences in the size of the head and position of the antennae from C. stygia. Cope's (1872a) account was republished twice (1872b; 1878).

Packard (1873) examined Cope's specimens of C. microcephala and synonymized the species with C. stygia. Examining specimens taken from a well in Orleans, Orange Co., Indiana, Packard ascertained that “the genus is not a member of the family Idotaeidae, but of the Asellidae, and the ‘egg-sacs’ are uropoda.” Packard retained Caecidotea as an entity separate from Asellus in placing Caecidotea in the family Asellidae, noting however that “it seems not difficult to recognize in Caecidotea an Asellus modified by its subterranean existence.”

Smith (1873) concurred with Packard (1873) that the external egg sacs figured by Cope (1872a) attached to C. microcephala originated from a lernaean copepod parasitic upon the blind fish found in the cave streams with Caecidotea. Smith (1873) also questioned the affinities of Caecidotea with Idotea. The following year Smith (1874) included C. stygia in a list of the freshwater crustaceans of the United States, listed the known collection localities, and repeated his conviction that Caecidotea “is evidently very closely allied to Asellus, and has no affinity with Idotea.” In 1875 Smith listed crustaceans described from Indiana and Kentucky caves, including Caecidotea stygia, and briefly discussed their origins. Smith could distinguish no difference between Caecidotea and Asellus except for the absence of eyes in Caecidotea.

Forbes (1876), in a list of Illinois Crustacea, synonymized Caecidotea with Asellus and redescribed Asellus stygius, using specimens of C. kendeighi collected in central Illinois. Although Forbe's redescription of Asellus stygius is relatively long and detailed, it is applicable entirely to C. kendeighi.

Hubbard (1880) found C. stygia to be common in pools in Mammoth Cave in the company of “a leech, or possibly worm” (see Kenk (1977) for notes on Sphalloplana occurring with Caecidotea). Hubbard included a dorsal habitus drawing of C. stygia and an enlarged view of the first antenna, noting the presence of esthetes and plumose setae.

Cope and Packard (1881) described Cecidotaea nickajackensis and compared it with Cecidotaea stygia.

Packard (1885a) compared the brains of Asellus communis and Cecidotaea stygia and found that the “eyeless Cecidotaea differs from the eyed form (Asellus) in the complete loss of the optic ganglia, the optic nerves, besides the almost and sometimes nearly total loss of the pigment cells and lenses.” Packard, who was a firm believer in the inheritance of acquired characters, attributed the losses to disuse in the absence of light.

Underwood (1886) included A. stygius in a checklist of American Crustacea and listed the range of the species as Indiana, Illinois, and Kentucky.

Packard's (1888) classic account of the cave fauna of North America included a redescription of Caecidotaea stygia and several other references to the species scattered through the paper. Packard pointedly disagreed with Forbes’ (1876) synonyomy of Caecidotea with Asellus. In resurrecting Caecidotaea, Packard provided a lengthy description and a number of illustrations for C. stygia, unfortunately using specimens from Illinois (C. kendeighi) and Pennsylvania (C. pricei) in addition to material from Indiana and Kentucky. Packard's drawings illustrate numerous appendages from C. stygia including the first figures of the pleopods 1 and 2, the latter too small to show detail.

In addition to the taxonomic consideration of Caecidotaea stygia, Packard (1888) listed records for the species from Illinois, Indiana, Kentucky, and Pennsylvania from caves, wells, springs, and drain tiles. Blind crayfish (see Hobbs and Barr (1972), Orconectes inermis) were reported to feed readily on C. stygia, although the food of the isopod was unknown. Packard's (1885a) paper concerning the eye and brain of Caecidotaea was reprinted in the later paper (Packard, 1888) with illustrations.

Hay (1891) cited Asellus stygius in Indiana from both wells and caves, noting the larger size of specimens taken from wells (i.e., C. kendeighi).

Garman (1892) discussed the origin of the cave fauna of Kentucky and stated his belief that animals found in caves were already partly adapted to subterranean life. Garman suggested that C. stygia existed before the formation of Mammoth Cave and probably did not originate in that cave.

Stebbing (1893) listed the range of C. stygia and noted that the absence of eyes did not distinguish it from Asellus.

In a lengthy paper on the crustaceans inhabiting subterranean waters in New Zealand, Chilton (1894) reviewed the work of Cope and Packard (1881) and briefly discussed an origin in “underground waters” rather than caves.

Packard's (1894) account concerning the origin of Caecidotea (Asellus) stygia consists of quotes from an earlier paper by Garman (1892).

Blatchley (1896) reported the collection of Caecidotaea stygia from several caves in Monroe, Lawrence, and Crawford counties, Indiana, during his 5-week horse-and-wagon trip exploring southern Indiana caves as state geologist. The cave from which Cope secured his specimens for the description of C. microcephala was identified as Saltpetre Cave, which is near Wyandotte Cave. The isopods were taken from troughs left in the cave by saltpeter miners.

Richardson (1901) included C. stygia in her key to isopods found along the Atlantic Coast with a new record for the species from Graham's Spring, Lexington, Virginia.

Hay (1902a) reported C. stygia from Mammoth Cave, where it was associated with Mancasellus (= Lirceus) macrurus, near the mouth of Echo River. In discussing the crustacean fauna of Nickajack Cave, Tennessee, Hay (1902b) reviewed the history of the genus Caecidotea, concluding that the validity of the genus was questionable, but retained the genus “on the grounds of convenience.” Hay also described Caecidotea richardsonae, comparing it with C. stygia and C. nickajackensis. With this comparison Hay included drawings of a C. stygia habitus, the pleotelson and uropods (showing a regenerated uropod), and pereopod 5.

Ulrich (1902) described C. smithii and compared it with C. stygia.

Richardson (1905) presented a key to the species of Caecidotea, including C. stygia. Collection records from Virginia, Kentucky, Indiana, and Illinois were repeated along with Hay's (1902b) drawings of C. stygia. A small drawing of the pereopod 1 by Richardson (1905, fig. 491c) appears to be of C. kendeighi. A lengthy redescription of C. stygia by Richardson includes the description of the propodus of the first pereopods as “armed on the inferior margin with two long triangular processes and three short ones.”

Banta (1907) published an extensive report on the fauna of Mayfield's Cave, Monroe Co., Indiana, making several interesting observations on the natural history of Caecidotea stygia. The isopod was found abundantly throughout the cave both on pool bottoms and the undersides of stones. On occasion C. stygia was seen crawling on stream banks and also outside of the cave under stones and dead leaves near the entrance. Both the amphipod Crangonyx gracilis and the blind fish Amblyopsis spelaeus were seen to prey upon C. stygia, and the isopod was noticeably less abundant in the areas of the cave where these species were found. Banta judged the food of the isopod to be decaying leaves, which he fed the animal successfully in the laboratory. Ovigerous females were taken “at different seasons, and quite small individuals are seen at all times, so that this species must breed throughout the year.”

Banta (1910a) used specimens of C. stygia from Mayfield's Cave in a study of the reactions of isopods (A. communis and C. stygia) to light. It was found that C. stygia is much less responsive to light than A. communis; is photokinetic and photonegative; is tolerant of light after lengthy exposure; is more responsive to light after periods of darkness. A second paper (Banta, 1910b) reveals C. stygia's great sensitivity to various sorts of mechanical stimuli (including water currents) and selectivity in food preference. Banta felt that C. stygia's photonegative and rheotactic responses would tend to keep the isopod from straying from subterranean to epigean habitats.

Stafford (1911) described C. alabamensis and compared it with C. stygia. Fowler (1912), following Richardson (1901), listed a single erroneous record for C. stygia. In his manual, Pratt (1916) included C. stygia: “the hand being armed with 2 long and 3 short teeth: central United States; in caves and deep wells.” Pratt apparently followed Richardson's erroneous description of the male 1st pereopod propus, actually based on specimens of C. kendeighi. The above was repeated by Pratt (1935) in the second edition of his manual, but with the species listed as A. stygia. Racovitza (1920), in a study of the male 1st pleopod, gave the position of insertion of the distal segment on the sympod in Caecidotea stygia as evidence that it is the exopod. In 1923 Racovitza pointed out the primitive state of the female pereopod 1 in Caecidotea stygia. In a study of the structure of the asellid 2nd antenna, Racovitza (1925) found that of Caecidothea (sic) stygia to be typical for the genus Asellus. Finally, in a posthumous work, Racovitza (1950) redescribed Asellus stygius in detail, using a male and a female from Mammoth Cave that he had obtained in an exchange with the National Museum of Natural History in 1912.

Chappuis (1927) listed Caecidotea stygia in his book on underground animals, repeating the localities given by Richardson (1905).

Giovannoli (1933a,b) mentioned the association of C. stygia with the amphipods Eucrangonyx gracilis and Crangonyx vitreus in the Mammoth Cave region.

Hoffman (1933) mentioned C. stygia and pointed out that almost nothing was known of the species’ biology. Miller (1933) analyzed the differences between Asellus and Caecidotea, using a table of the species that included C. stygia, and synonymized Caecidotea with Asellus.

Van Name (1936) included Caecidotea stygia in a list of species of American asellids. The morphology of the species was discussed, and illustrations of C. kendeighi taken from Richardson (1905) repeated. Van Name followed various authors in listing the range of the species through Indiana, Kentucky, Illinois, Virginia, and Pennsylvania. Caecidotea stygia was also compared with C. alabamensis and C. nickajackensis. Van Name (1942) quoted a statement from Mackin and Hubricht (1940) regarding the taxonomy of C. stygia.

In a laboratory guide (Park et al., 1939), C. stygia appears in a key and as “cave isopod” in a list of suggested experiments with cave animals.

Mackin and Hubricht (1940) described 7 new species of asellids, retaining with a degree of reluctance the use of the genus Caecidotea. The morphology of C. stygia was compared to C. dimorpha and C. oculata, and differences in food preferences of C. stygia, C. packardi, and C. spatulata noted.

Dearolf (1942) added another Mammoth Cave collection record for Asellus stygia.

Chappuis (1950) recognized Caecidotea as a subgenus of Asellus and reported on Bolivar and Jeannel's collections of C. stygia from Mammoth Cave, Reid's Cave, and Horse Cave, Kentucky, and Marengo Cave, Indiana. He illustrated the marked changes that take place in the male pereopod 1 during growth and stated that determination of species of Asellus must be based on the male pleopods 1 and 2, since their morphology remains constant during growth.

In a checklist of Ozark cave invertebrates, Hubricht (1950) apparently accepted the synonymy of Caecidotea with Asellus, listing “Asellus stygeus (Cope). Found in caves east of the Crystal City Escarpment.” This notation is evidently a lapsus since the species had been listed by Mackin and Hubricht (1940) correctly spelled as Caecidotea stygia Packard (not Cope).

Birstein (1951) discounted the genus Caecidotea, partitioned Asellus into subgenera, and placed C. stygia in the subgenus Conasellus.

Dearolf (1953) listed collection records of Caecidotea stygia from 2 Kentucky caves and Marvel Cave, Missouri. Pennak (1953) included Asellus stygius in a key to the Isopoda, listing the range as “Mo., Ind., and Ky.,” and included a habitus drawing of A. stygius that bears a strong resemblance to C. kendeighi. Bresson (1955) listed the range of A. stygius as Illinois, Indiana, Kentucky, and Virginia in a checklist of nearctic asellids. In recording a new collection record from Kentucky, the species was placed in the subgenus Proasellus.

Chappuis (1955) mentioned Asellus stygius in discussing asellid genera and again (Chappuis, 1957) in describing Asellus condei. Cole (1959) noted the occurrence of A. stygius in “caves and wells in Kentucky, Tennessee, and Indiana.” Mackin (1959) listed A. stygius in a key and checklist to U.S. isopods. Nicholas (1960a) included Caecidotea stygia in a checklist of U.S. troglobites, listing the range as Kentucky, Indiana, Pennsylvania, and Virginia. In the same year he (1960b) again listed C. stygia as widespread throughout Pennsylvania. Barr (1961) reported A. stygius from the upper Cumberland region of Tennessee.

Minckley (1961) discussed in some detail the occurrence of Asellus stygius in epigean habitats. The species was collected from spring-fed streams where it was a minor but consistent member of the fauna.

In a checklist of Virginia troglobites, Holsinger (1963) indicated that records of C. stygia in Virginia and Pennsylvania were erroneous. More recently (Holsinger, 1976), he again stated that Pennsylvania records are incorrect. Minckley (1963) again noted the presence of A. stygius in a surface spring-fed stream (Doe Run, Meade Co., Kentucky). Poulson (1963) observed that for specimens of Amblyopsis spelea longer than 45 mm, Asellus stygius is a major food item.

Steeves (1963a) partly redescribed and illustrated Asellus stygius and listed a number of collection localities in Kentucky, Indiana, and Tennessee along with a single record from “Illinois: wells, Monroe Co.” Steeves established a Stygius group and placed within it 4 species. Asellus stygius was believed by Steeves to have closest affinities with A. bicrenatus. In a separate paper, Steeves (1963b) mentioned A. stygius without adding new information.

Citing Packard (1888), Poulson (1964) reported “the isopod Asellus stygius (83) has pigmented eyes with lenses and retinal cells when it lives in wells but lacks these when it lives in caves, even though the optic ganglia and nerves are absent in both cases.” This is a reference, in part, to C. kendeighi. Poulson also cited data from Banta's (1910a, 1910b) experiments with A. stygius.

Vandel (1964, 1965) cited Asellus stygius as an example of an ancient species of Asellus, because it has not developed the “crochet nuptual” on the male 4th pereopod.

Eberly (1966) noted the common occurrence of Asellus stygia in Indiana caves in his description of Asellus jordani. Steeves (1965) described A. barri and indicated its affinities to A. stygius. Clifford (1966) reported A. stygius as a minor component of the aquatic fauna in an ecological study of a small stream in Caldwell Hollow, Brown Co., Indiana, where it lives in ground water discharged into the hollow's stream.

Krekeler and Williams (1966) included A. stygius in a checklist of Indiana cave animals. Mohr and Poulson (1966) illustrated a typical section of the stream ecosystem in Upper Twin Cave, Lawrence Co., Indiana, including the role of isopods (C. stygia) as prey for the fish, crayfish, and flatworms in the cave.

Steeves (1966) attempted to show the affinities of a number of troglobitic asellids, including A. stygius, with a dendogram of 13 asellid pleopod 2 endopod tips arranged in a pattern illustrating proposed lineages. In this evolutionary scheme A. parvus preceded A. alabamensis, from which arose directly A. richardsonae, A. nortoni, A. stygius, and A. antricolus via 4 separate lineages. Asellus stygius is shown to give rise to A. recurvatus and A. barri. Asellus antricolus is partly redescribed in this paper, and collections previously identified as A. stygius are noted.

Barr (1967) discussed briefly the presence of A. stygius in the Mammoth Cave System and noted some of its zoogeographic affinities. The following year Barr (1968) suggested that the food of A. stygius consists of bacteria and fungi and cited Banta's (1910b) report on the sensitivity of this isopod to tactile stimuli.

Minckley and Cole (1968) compared the collection locality of Speocirolana thermydronis to the epigean habitat of A. stygius discussed by Minckley (1961). Steeves and Holsinger (1968) also cited the paper by Minckley (1961), noting the possibilities for dispersal conferred by A. stygius's ability to survive in an epigean habitat. Holsinger (1969) again noted the vagility of A. stygius in its apparent ability to migrate under the Mississippi River using subfluvial channels and compared the isopod's range to that of the amphipod Apocrangonyx subtilis.

Seidenberg (1969) cited the data of Banta (1907; 1910a; 1910b) concerning A. stygius.

Steeves (1969) mentioned Caecidotea stygia in discussing the origin and affinities of Appalachian cave asellids. Henry and Magniez (1970) included C. stygius in a list of species of Conasellus.

Williams (1970) noted that although A. stygius had been reported from epigean habitats, the species is typically subterranean and did not fall within the scope of his revision of epigean asellids.

Holsinger and Steeves (1971) relegated Richardson's (1905) Virginia record of C. stygia to synonomy with Asellus pricei. Cooper and Beiter (1972) reported A. stygus and other crustaceans as food of the southern cavefish Typhlichthys subterraneus. Fleming (1972a) compared Asellus stygius with A. extensolingualus and listed new localities for the species. Fleming (1972b) also compared A. stygius with A. paurotrigonus. Hobbs and Barr (1972) recalled Packard's (1888) reference to Caecidotea as the food of the blind crayfish Orconectes inermis inermis in Indiana. Fleming (1973) included Asellus stygius in a key to the species of the genus. Hobbs (1973a) reported a single cavernicolous species of asellid, Asellus stygius, from Indiana in a checklist of the state's cave fauna. Hobbs (1973b) also reported an unidentified isopod from caves in the Lost River karst area of southern Indiana.

Page (1974) included Steeves’ (1963a) Monroe Co., Illinois, record in a checklist of Illinois Malacostraca. Barr and Kuehne (1971) analyzed the aquatic ecosystem of the Mammoth Cave system in detail, including A. stygius. Craig (1975) followed Fleming's (1972a) record of A. stygius for Missouri. Holsinger (1976) noted Nicholas's (1960a) error in reporting A. stygius in Pennsylvania. Hobbs et al. (1977) again noted the appetite of the crayfish Orconectes inermis inermis for Caecidotea.

Bowman and Beckett (1978) reported new localities of Caecidotea stygia from Ohio and redescribed the species using specimens from the Cincinnati region.

Peck and Lewis (1978) recorded Caecidotea stygia from 3 caves in southeastern Illinois and discussed the zoogeography of this and other subterranean invertebrates in Illinois.

MATERIAL EXAMINED.—ILLINOIS. Hardin Co.: Cave Spring Cave, 2 mi NW Rosiclare, leg. E. Lisowski and D. Osterbur, 5 Jun 1976, 3, 1 (INHS); leg. S. Peck, 24 Oct 1965, 3, 6. Layoff Cave, Rosiclare, leg. J. Lewis, 17 Feb 1974, 5, 9 26 Oct 1974, 3, 1 leg. J. Lewis and M. Meister, 21 Jun 1977, 4, 6, 14 juv.; leg. S. Peck, 24 Oct 1965, 2, 2. Griffith Cave, leg. S. Peck, 2, 2. Johnson Co.: spring 2 mi S Forman, leg. J. A. Boyd and L. M. Page, 27 Apr 1976, 3, 1.

DIAGNOSIS.—[Since C. stygia was redescribed in detail recently (Bowman and Beckett, 1978), we give here a diagnosis rather than a full description.] Eyeless, unpigmented, length up to 16 mm. Antenna 1 esthete formula 3–0–1 or 4–0–1. Palm of gnathopod without proximal process, but defined by 1 to several spines; mesial process acute or subacute; distal process close to mesial process, rounded or bicuspate. Palmar processes may be rudimentary or absent in having differentiated pleopods 1 and 2. Pereopod 4 identical in and . Pleopod 1 of with 3–5 retinacula; exopod truncate distally, with a few short setae on medial part of distal margin; distolateral corner produced into rounded spinulose lobe; lateral margin straight to slightly concave, armed with setae increasing in length proximally. Endopod tip of pleopod 2 with slender, pointed, nearly straight cannula, sometimes slightly bent at tip; lateral process slender, digitiform, curved toward cannula; caudal process low, broadly rounded. Pleopod 4 exopod, type B, with spines on proximal part of lateral margin. Uropod rami linear.

ETYMOLOGY.—Not given, but obviously meaning “of the river Styx,” referring to the habitat in underground waters.

RELATIONSHIPS.—Several species of Caecidotea share the following combination of characters with C. stygia: gnathopod with mesial and distal processes, but without proximal process; pleopod 1 exopod with short distal setae and longer lateral setae; pleopod 2 endopod tip with 2 conspicuous processes, cannula and lateral process; pleopod 4 exopod type B. These species include C. meisterae, C. whitei, and perhaps C. barri, C. bicrenata, and C. franzi, in which pleopod 4 has not been described. Of these species, C. meisterae and C. whitei differ in lacking spines on pleopod 4 and in having uninterrupted esthete series on antenna 1. In C. barri the pleopod 1 exopod resembles that of C. stygia in having a truncate distal margin, but the endopod tips of pleopod 2 of C. barri and also C. franzi are quite different. The relationships of C. stygia cannot be clearly elucidated at the present time.

HABITAT.—Caecidotea stygia is primarily an inhabitant of limestone caves, where it is found typically in streams, drip pools, or rimstone dam pools. In these habitats C. stygia is often abundant and may be found clinging to the undersides of dead wood or rocks, or crawling across stream or pool bottoms. This species is sometimes reported from springs (Minckley, 1961) or epigean streams, but its occurrence in these habitats is usually the result of being flushed out of caves or other subterranean habitats.

RANGE.—Fleming (1972a) reported Asellus stygius from Griffith, Layoff, and Cave Spring caves in Hardin County. These caves are discussed by Bretz and Harris (1961). Griffith and Cave Spring caves are both developed in the Mississippian Fredonia member of the Ste. Genevieve Limestone (Bretz and Harris, 1961; Willman et al., 1975), which lays beneath the Rosclaire Sandstone, and presumably Layoff Cave developed in the same member, although this was not discussed by Bretz and Harris. The Johnson County locality is also in a region where Mississippian limestones prevail, possibly the Ste. Genevieve, and illustrates the ability of this species to disperse westward from the central part of its range past a rather extensive fault zone in Hardin, Pope, and Johnson counties. Preliminary observations of the distribution of C. stygia in Illinois by Peck and Lewis (1978) suggested that the fault zone had prevented westward dispersal, but this suggestion no longer appears tenable.

The Illinois localities of Caecidotea stygia lie within the Interior Low Plateaus Province, as do most of the localities for this species in Indiana, Kentucky, and Tennessee; however, in Indiana and Ohio, this species occurs in caves and groundwater discharges within the Central Lowlands Province, as reported by Bowman and Beckett (1978).